Help designing a latching electronic switch

In your schematic:

GND 1 2 3 4 5 6 7 8 | | | | | | | | | | _/ | | | | | | | | |-o/ o-o-)-)-)-)-)-)-)------o | | | | | | | | | | _/ | | | | | | | | o-o/ o---o-)-)-)-)-)-)--\\ | __ | | | | | | | \\ o-| \\ __ | _/ | | | | | | \\ | )--| \\ __ o-o/ o-----o-)-)-)-)-)-\\ \\o-|__/ | )--| \\ __ | | | | | | \\-----------|__/ | )--| \\ | _/ | | | | | /---------------|__/ | )-o o-o/ o-------o-)-)-)-)-----/ o--------------------|__/ | | | | | | | | | _/ | | | | | o-----------------------o o-o/ o---------o-)-)-)--------o | __ | | | | o-| \\ __ | _/ | | | | )--| \\ __ o-o/ o-----------o-)-)-----------o-|__/ | )--| \\ | | | o-|__/ | )---- | _/ | | | o-|__/ o-o/ o-------------o-)------------------o | | | | | _/ | | o-o/ o---------------o-------------------------o

the assumption is that the leads going to the XBOX (1-8) are pulled up to Vcc through resistors, since the circuiit wouldn't make much sense otherwise. That is, closing the switches would have no effect on the logic if the leads went to ground, and they're not allowed to float. That is, unless they're bipolar TTL, but that's another story and is bad practice and they'd wind up looking like they were pulled up anyway.

If the inputs are pulled up to Vcc and no switches are pressed, then the output of the circuit will go high since, for an AND, the output goes high when all the inputs are high. Consequently, if _any_ of the switches is pressed, that input will go low and so will the output. Therefore, since you want the output state to change when _all_ the switces are pressed, you need something with and output which will change when _all_ the inputs are low, and that's an OR. (or a NOR, depending on which way you want the output to swing when all the inputs are low.

Having said that, the PDF I posted on abse shows an 8 input NAND being used, the reason being that with the SPDT switches you showed being normally connected to ground, all of the swithes being pressed will result in all of the inputs of the NAND going high, causing the output to go low, which is what I need to start the chain of events which will result in the toggle.

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John Fields
Professional Circuit Designer
Reply to
John Fields
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The 8 input NAND is a 74HC30 (with various prefixes or suffixes), the '74 is a flip-flop.

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Peter Bennett VE7CEI 
email: peterbb4 (at) interchange.ubc.ca        
GPS and NMEA info and programs: http://vancouver-webpages.com/peter/index.html 
Newsgroup new user info: http://vancouver-webpages.com/nnq
Reply to
Peter Bennett

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The 8 input NAND is an SN74HC74AN, (TI) and the flip-flop can be
either an SN74HC74AN, (TI) an MC74HC74AN, (ON Semi) or a 74HC74N
(Philips).  Go for the cheapest, they\'ll all work the same in your
circuit.
Reply to
John Fields

Hi John

I've found the following on the RS Components site but I'm not sure if they're the right ones.

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Do you know which of the flipflops I need?

Thanks Mark

Reply to
Mark Ashley

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Yup! 

Thanks.
Reply to
John Fields

make a drawing at

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( a 100dpi scannerdrawing will do fine ) ; go to the mailbox the ID is 'schetsen' and the password is 'diverse'. send the drawing to the same mailbox 'schetsenATyahoo.com' and I'll see what I can do for you ( for free of course )

Reply to
William

make a drawing at

formatting link
( a 100dpi scannerdrawing will do fine and wouldn't be to big ) ; go to the mailbox the ID is 'schetsen' and the password is 'diverse'. send the drawing to the same mailbox 'schetsenATyahoo.com' and I'll see what I can do for you ( for free of course )

Reply to
William

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Didn\'t notice it the first time around, but the sense of the power-on
RESET is wrong.  The way it\'s shown the 4013 will stay in RESET as
long as power is applied.
Reply to
John Fields

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Why don\'t you read the entire thread and any binaries posted to abse
and then, if you have someting to say, post your comments here for
everyone to see?
Reply to
John Fields

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